1. Technical Field
The present invention relates to an oscillation circuit and electronic equipment with the oscillation circuit built therein, and particularly to an oscillation circuit suited to equipment performing communications through Frequency Shift Keying (FSK) and electronic equipment with the oscillation circuit built therein.
2. Description of the Related Art
As a method of communicating in digital signals, a communications method through FSK has hitherto been known. FSK is, for example, a modulation method of discretely changing an instantaneous frequency of a carrier wave corresponding to digital signals of 0 and 1. This makes it possible to apply modulation while keeping an amplitude change at a constant level, subject to very little effects due to level fluctuation and noise. Further, there are characteristics of simple circuit configuration and a wider band width than Amplitude Shift Keying (ASK) and Phase Shift Keying (PSK).
As mentioned above, the communications method through FSK has a high utilization value, and oscillation circuits and oscillators using this type of communications method have been variously proposed. For example, the oscillation circuits and the oscillators proposed in Japanese Patent No. 2925158 and Japanese Published Unexamined Patent No. 2004-40421 make use of the communications method of FSK.
The oscillator described in Japanese Patent No. 2925158 is constituted such that a plurality of SAW resonators of different resonant frequencies are provided on a piezoelectric substrate, and that the same number of oscillation circuits as SAW resonators are provided on a semiconductor integrated circuit substrate, while the oscillation circuit is connected to a change-over circuit.
In the oscillator of such configuration, a change-over circuit is operated according to a band of a frequency in use, an oscillation circuit connected to a SAW resonator corresponding to a desired frequency band is selected, and oscillation is generated.
Further, the oscillation circuit described in Japanese Published Unexamined Patent No. 2004-40421 is provided in the circuit with SAW filters of two different types of frequency bands which are permitted to pass, with the change-over circuit on an output side of the SAW filters, thus constituting a feedback circuit equipped with an amplifier and a fixed phase shifter.
In such oscillation circuit, it is possible to select only a frequency of a desired frequency band by changing over a SAW filter connected by the change-over circuit.
According to the oscillators and the oscillation circuits described in the above-mentioned Japanese Patent No. 2925158 and Japanese Published Unexamined Patent No. 2004-40421, an oscillator and an oscillation circuit corresponding to a design frequency band can be provided.
However, the oscillators and the oscillation circuits described in the above-mentioned Japanese Patent No. 2925158 and Japanese Published Unexamined Patent No. 2004-40421 are constituted such that a plurality of SAW resonators or SAW filters of different resonant frequencies are respectively provided, which correspond to a required frequency band by changing over a frequency band in use. Consequently, when changing over SAW resonators or SAW filters, an oscillating frequency may be instantaneously cut off as a result of factors due to electrical connection, factors due to a phase shift difference in SAW resonators and SAW filters, and the like, thereby causing a discontinuity of output phase shifting and generating an error at the time of change-over. Such problems exert considerable effect upon high-speed communications. Further, in the oscillation circuit described in Japanese Patent No. 2925158, by providing a plurality of SAW resonators of different specifications within one circuit, there is a problem which makes it impossible to obtain frequency precision and deviation precision in respective SAW resonators.
It is an object of the present invention to provide an oscillation circuit and electronic equipment with the oscillation circuit built therein, which can solve the above-mentioned problems and, even in a case where the frequency band changes, eliminate discontinuity of output phase shifting accompanying the change-over of the frequency band, and properly maintain the frequency precision and the deviation precision as well.
To accomplish the above-mentioned object, with any of the SAW resonators changing over the frequency band as a reference, it is conceived proper if a phase shift at the time of change-over can be instantly made to correspond to a signal of the SAW resonator set as the above-mentioned reference. Also, to keep the frequency precision and the deviation precision properly, it is considered necessary to reduce mutual interference among the SAW resonators performing the change-over.